Well, that´s a relief. I thought everyone understands this but I don´t.

But...a while ago you corrected me, tunci:

Onlardan biri içine bakıp diğerine “Bu güzel serin bir yer gibi görünüyor.

I had replaced "ın" with "dan" , it is not because it was wrong, it is because it didn´t sound good to me in that particular sentence.

The smoothness of flow in sentences is important too.

Clear. Thanks, tunci. Your brain seems to be in good oxygen again.

I need to point out one more thing before my oxigen level goes down again ; That is ;

 in the sentence we should have added the indirect object [ kuyu ] as without it, it sounds bit lacking in the meaning. I know in the context we will tell it but still not sounding complete without indirect object.

by looking inside BUT looking in  where [inside of what ]?

Therefore we´d better add it ;

Onlardan biri kuyunun içine bakıp diğerine “Bu güzel serin bir yer gibi görünüyor.

One last thing ; We can drop the first part [head] and just say "biri" as it is more practical and easy way.

Biri, kuyunun içine bakıp diğerine “Bu güzel serin bir yer gibi görünüyor.

I´d better head to my bed now..

Second form implies a selection

Çocukların biri = a member of children

Çocuklardan biri = a (selected) member of children

Or using set theory terms:

Çocukların biri = "biri" is a member of the children set

Çocuklardan biri = "biri" is a member of a subset of the children set (a subset implies a selection from a set)

Thank you, too, si++. I guess I can imagine what you mean. There is a similar slight difference in my native language, made with ablative also.

Ablative is one interesting case.

In onlardan birçoğ|u the possessive suffix in the second part can´t be motivated by a possessive construction. There has to be another reason.